1. Field of the Invention
This invention relates generally to microwave devices and in particular this invention relates to a lightweight distributed parameter microwave filter having resonator rods made of a combination of a metal having an extremely small coefficient of thermal expansion and a lightweight metal alloy which are housed in a lightweight metal housing.
2. Description of the Prior Art
Microwave filters utilizing resonator rods are well known in the prior art. Both interdigitated resonant rod and "comb-line" resonant rod arrangements have been used. The resonant frequency of a housing may be changed by changing the capacitance or the inductance of a resonant element. A housing made of a metal having a relatively large coefficient of thermal expansion would cause a large change in the resonant frequency as the ambient temperature changes. And, a housing made of metal having a relatively small coefficient of thermal expansion would not cause the resonant frequency to change greatly as a result of the ambient temperature change. Similarly, a resonant element having a relatively high coefficient of thermal expansion would cause a significant shift in resonant frequency if the rod is subjected to widely varying ambient temperature. The end of the resonator element and the interior housing wall provide a tuning capacitance. If the spacing between the resonator element and the housing changes due to different coefficients of thermal expansion, so too, does the resonant frequency change. Thus, as the rod changes in length and as the tuning capacitance changes the resonant frequency also changes. Therefore, the resonant frequency is dependent upon the coefficient of thermal expansion.
Generally, for wide temperature applications invar, a combination of nickel and iron, is used for the resonant rods and the housing because of its extremely low coefficient of thermal expansion which is 1 .times. 10.sup.-6 in/in/.degree. C. The dimensions of an invar housing and the length of the resonator rods located therein are constant and the filter provides a very stable output frequency over a broad temperature range due to the low coefficient of thermal expansion. On the other hand, aluminum has a relatively high coefficient of thermal expansion of 22 .times. 10.sup.-6 in/in/.degree. C. A variance in ambient temperature would cause a relatively large variation in the dimensions of an aluminum microwave housing and the length of the resonator rod resulting in a relatively large frequency shift.
Using materials having a coefficient of thermal expansion which is greater than invar causes the output bandpass frequency to shift as the filter experiences dimensional changes as a result of temperature fluctuations. As the housing changes in size so does the resonant frequency. As the reactive rods change in size as a result of temperature variations so does their resonance change. Also, the capacitance between the walls of the housing and the reactive rods change. Thus, the thermal expansion and contraction of materials with relatively higher coefficient of thermal expansion causes several mechanisms to affect the resonant frequency of the filter.
It is not always practical nor efficient to use filters made of invar in space applications because of the relatively high weight of such a device. On the other hand, a microwave filter utilizing only a lightweight metal such as aluminum has the disadvantage of frequency shift with temperature change.
A more detailed discussion of microwave filters may be found in "Principles and Applications of Waveguide Transmission" by George C. Southworth at pages 285 to 306.